APPLICATION FOR RHIZOBACTERIA IN TRANSPLANT PRODUCTION AND YIELD ENHANCEMENT

PGPR (plant growth-promoting rhizobacteria) are root-colonizing bacteria that benefit plants by increasing plant growth or reducing disease. Current applications of PGPR as biocontrol agents rely on mixtures of PGPR as components in integrated management systems in which reduced rates of agrochemicals and cultural control practices are used. The finding that some strains of PGPR can elicit systemic disease protection has renewed interest in PGPR for practical application in agriculture and horticulture. We report here results of attempts to combine PGPR with different modes of action with organic amendments. Our hypothesis was that such an integrated system could be used for transplanted vegetables to produce more vigorous transplants that would be tolerant of nematodes and other diseases for at least a few weeks after transplanting to the field. The specific combination that we tested consisted of Bacillus subtilis strain GB03, B. amyloliquefaciens strain IN937a, and B. subtilis strain IN937b together with chitosan. Strain GB03 produces antibiotics while IN937a and IN937b elicit induced systemic resistance. Chitosan was added to stimulate a microflora antagonistic to nematodes. Results demonstrated that the combination of two bacilli strains with chitosan resulted in significant growth promotion that was correlated with induced resistance in tomato (Lycopersicon esculentum), bell pepper (Capsicum annuum), cucumber (Cucumis sativus) and tobacco (Nicotiana tabacum). The preparation has been commercialized by Gustafson, LLC under the name “BioYield” and is discussed as a model for extending PGPR technologies to growers. BioYield is incorporated into the potting mix used to prepare transplants. Treated transplants demonstrate increased shoot and root growth, enhanced stem diameter, less transplant shock, and rapid development of new roots. Disease protection is sometimes observed, but the most reproducible effect is growth promotion resulting in yield increases with many tested transplant systems. INTRODUCTION Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion (Cleyet-Marcel et al., 2001; Kloepper, 1994; Glick, 1995). Inoculation of crop plants with certain strains of PGPR at an early stage of development improves biomass production Proc. XXVI IHC – Transplant Production and Stand Establishment Eds. S. Nicola, J. Nowak and C.S. Vavrina Acta Hort. 631, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 218 through direct effects on root and shoot growth. Several reviews discuss specific aspects of growth promotion by PGPR (Cleyet-Marcel et al., 2001; Glick, 1995). Inoculation of ornamentals, forest trees, vegetables, and agricultural crops with PGPR may result in multiple effects on early-season plant growth, as seen in the enhancement of seedling germination, stand health, plant vigor, plant height, shoot weight, nutrient content of shoot tissues, early bloom, chlorophyll content, and increased nodulation in legumes. Biological control using introduced PGPR against plant diseases has been extensively studied under greenhouse and field conditions. PGPR exhibit biological control via several mechanisms. Siderophores, antibiotics, hydrogen cyanide, and cellwall degrading enzymes are among the metabolites produced by PGPR that reduce growth or activity of the pathogen. Biological control may also result from direct interactions between PGPR and the host plant. In this case, host disease defense reactions are stimulated, a process termed induced systemic resistance (ISR). Further, several studies shave shown that individual strains of PGPR elicit ISR against multiple pathogens bacteria, fungi, and viruses on one host plant, including tomato, bell pepper, and cucumber (Raupach et al., 1996; Raupach and Kloepper, 1998; 2000; Reddy et al., 1999; Reddy et al., 2000; Jetiyanon and Kloepper, 2002). To date, little research has been conducted to determine whether PGPR strains can elicit ISR in a range of host plants. Improving the consistency of beneficial effects is a goal for PGPR research and development. Most approaches for biocontrol of plant diseases and plant growth promotion have used applications of single PGPR strains. Because one strain is not likely to be active in all soil environments or against all pathogens that attack the host plant, the use of a single strain may partially account for the reported inconsistent performance by PGPR. Supporting the use of a mixture of PGPR strains are two studies on ISR (Raupach and Kloepper, 2000; Jetiyanon and Kloepper, 2002) in which mixtures of PGPR provided greater activity against a broader range of plant pathogens than did single strains. Historically, PGPR have been applied as seed treatments to row crops. In our efforts to apply mixed PGPR inoculants to vegetables, we have been working on applying PGPR in the growing media used to prepare transplants. Our study was conceived several years ago from a group effort to integrate applications for vegetable transplants. The rationale was to combine PGPR with organic amendments, as some organic amendments, including chitin, can be mixed into agricultural soils with the effect of reducing nematode damage to plants (Rodriguez-Kabana, 1986; Hallmann et al., 1998; Suganda, 1999). We previously tested organic amendments of chitin, pine bark, and hemicellulose together with phytochemicals for effects on tomato transplant growth and root-knot nematode severity (Kokalis-Burelle et al., 2002a). Among the tested organic amendments, only treatments containing chitin increased plant root weight and reduced nematode galling compared to the control. The specific goal of this project was to determine if an integrated biological preparation could protect vegetable transplants against diseases for several weeks after being transplanted into the field. The broader purpose was to accelerate development of vegetable transplant plugs and to increase plant health. MATERIALS AND METHODS PGPR Strains Three spore-forming, bacilli PGPR strains were used: Bacillus subtilis strain GB03, B. amyloliquefaciens strain IN937a, and B. subtilis strain IN937b. Strain GB03 has shown biological control activity against Rhizoctonia solani and Fusarium spp. (Backman et al., 1997) and is part of a commercial product, Kodiak (Gustafson LLC, Dallas, Texas, USA) that is used as a seed treatment of cotton. Strains IN937a and IN937b elicit ISR in cucumber against cucurbit wilt caused by Erwinia tracheiphila, anthracnose caused by Colletotrichum orbiculare, and mosaic disease caused by cucumber mosaic virus (Raupach and Kloepper, 1998; 2000; Zehnder et al., 2000). These strains also elicit ISR in tomato against bacterial speck caused by Pseudomonas syringae pv. tomato, cucumber